Internet of Things, IoT, is expected to increase the number of connected devices significantly. A vast majority of these devices will likely operate in unlicensed bands, in particular the 2.4 GHz ISM band. At the same time, there is also increased demand for using the unlicensed bands also for services that traditionally have been supported in licensed bands. As an example of the latter, third generation partnership project, 3GPP, that traditionally develop specifications only for licensed bands have now also developed versions of Long Term Evolution, LTE, which will operate in the 5 GHz unlicensed band.
Many times, the transmission to these kinds of devices will not be only to one specific device, but rather the transmission will be intended to several or even all devices associated with the network node. These kinds of transmissions referred to as multi-cast and broadcast, respectively, are in at least some aspects more challenging than unicast transmissions. One such challenge is that the different receivers for a multi-cast/broadcast signal typically experience very different receiver conditions. It is not really possible to optimize the transmission for each receiver individually. This implies that if the transmitted signal uses a high modulation and coding scheme, MCS, to obtain good spectrum efficiency, devices with poor receiver conditions will not be able to correctly receive the packet. Conversely, if a packet is sent with low MCS, which is robust, in order to ensure that as many devices as possible are able to decode the packet, those receivers that have favorable channel conditions will need to be receiving, i.e., be awake, for an unnecessarily long time, resulting in unnecessarily high power consumption. Since having some devices not receiving the packet is typically worse than having some devices using some additional energy, the most robust MCS is typically used.
In a situation with a transmission to only one receiver, the MCS is typically optimized by finding as high MCS as possible which can be received with not too high error rate, i.e., maximizing the throughput. The finding of the optimal, or at least suitable, MCS is commonly referred to as link adaptation, LA, LA can be done in many ways, and with or without explicit feedback from the receiver. Regardless how the LA is done, it does require some kind of feedback. Therefore, if no such feedback is available, one may also have to resort to the same approach as used in multi-cast and broadcast, namely to use the most robust MCS.
Multicasting and broadcasting are typically done by using the most robust MCS in order to ensure that as many devices as possible are able to receive the message. This is not energy efficient because the total time that all the receiving devices need to be awake becomes very large. Also in case the MCS is determined by the link requirements of the device with the lowest SNR, the devices with much better SNR will need to be receiving for a longer time than the channel conditions actually allow. The reason is that the MCS used to modulate and code the packets must be robust enough so that the device with the weakest link and lowest SNR can decode it, and the packet length is directly related to the MCS. Since energy efficiency is very important in battery operated IoT devices, these approaches are not well suited.
The fact that different devices have different channel conditions can also be explored when forming the transmitted signal. As an example, in Digital Video Broadcasting for Terrestrial, DVB-T, which is standard for terrestrial broadcasting of e.g. TV programs in Europe, hierarchical modulation is used. In ETSI EN 300 744 “Digital Video Broadcasting, DVB; Framing structure, channel coding and modulation for digital terrestrial television” this is described in detail.
The idea with hierarchical modulation is to make some of the bits significantly more robust than the others by using a non-uniform signal constellation. An example of such a non-uniform constellation is depicted in FIG. 5, which is taken from the above mentioned standard. In this example the two first bits are unique for the respective quadrant, whereas the two last bits contains information about which one of the points within a quadrant. Clearly, the probability of a bit error will be lower for the two first bits than the two last ones. In DVB, the reason for having this kind of modulation was that some receivers, e.g., those implemented in a portable device with a small antenna with arbitrary direction can be expected to receive a much weaker signal than a receiver with a roof-top mounted antenna, which would be typical for a typical TV receiver in a home. However, the screen for a portable device may be, say, 5″, whereas a TV at home may be of the size of 42″ or even more. Therefore, the screen resolution required for a portable device typically is substantially less than what is needed for a home TV. Now as the receiver conditions are so fundamentally different due to the different antenna arrangements, one can transmit a low resolution picture using the most robust bits, and then complement this to obtain a high resolution picture by sending additional information using the less reliable bits. Since a large screen also is connected to a roof-top antenna, there is a good match.
Another technology which is based on similar ideas is Semi Orthogonal Multiple Access (SOMA). In SOMA, the fact that different receivers may experience very different conditions is explored. Essentially, in SOMA the more robust bits are used for transmission to a receiver with poor channel conditions, whereas the less robust bits are used for transmission to a receiver with good channel conditions. The receiver with poor channel condition then only use the more robust bits; it may not even be aware of that more information is transmitted. The receiver with good channel conditions, on the other hand, first decode the more robust bits, e.g. determine in what quadrant the signal constellation point is, and then once this knowledge is obtained, decode the remaining bits.
In US2015326360 there is disclosed a method for Non Orthogonal Multiple Access (NOMA) implementing hierarchical modulation. The method involves superposing enhancement modulation layer (EL) on base modulation layer (BL) and transmitting the result. The signal is transmitted to a plurality of User Equipments (UE) where the UEs with good reception can decode both layers. UEs with low reception quality can only decode the BL. It is possible to resend the EL only by using Hybrid Automatic Repeat Request (HARQ).
In WO2010039013 there is disclosed a method for coded relay cooperation with hierarchical modulation. The method involves transmitting the same layered information using relays and over several time slots. The method involves dividing a packet into a number of small blocks based on priority and modulated using hierarchical modulation
In EP1211837 there is disclosed a method for transmitting packets with high error protection followed by packets with low error protection.
Despite the various efforts to overcome the problem with transmission to receivers with varying channel conditions there is still room for improvement. For example, if one were to find mechanisms that would alleviate the problems with broadcasting/multicasting to devices with different receiving capabilities it would enable a reduction of unnecessary signaling and detecting and hence reduce the energy consumption.